Quarks & Leptons
Quarks
- Quarks are fundamental particles that make up other subatomic particles such as protons and neutrons
- Protons and neutrons are in a category of particles called hadrons
Hadrons are defined as any particle made up of quarks
- Fundamental means that quarks are not made up of any other particles. Another example is electrons
- Quarks have never been observed on their own, they’re either in pairs or groups of three
- There are six flavours (types) of quarks that exist:
The six flavours of quarks
- The charge of a hadron is determined by the sum of the charges of its quarks
- Each flavour of quark has a certain relative charge:
Each flavour of quark has a charge of either +⅔e or -⅓e
- For example, a proton is made up of two up quarks and a down quark. Adding up their charges gives the charge of a proton:
+⅔e + ⅔e - ⅓e = +1e
- The equivalent antiparticle of the quark is the anti-quark
- These are identical to quarks except with opposite relative charges
Each flavour of anti-quark has a charge of either -⅔e or +⅓e. The quark composition of anti-protons and anti-neutrons changes to anti-quarks
- Quarks have a baryon number of +1/3
- Anti-quarks having a baryon number of –1/3
- Strange quarks have a strangeness of –1
- Anti-strange quarks have a strangeness of +1
- This is unique to the strange quark
Worked example
Particles are made up of a combination of three quarks or two quarks. Which quark combination would not give a particle a charge of -1 or 0?
A. up, strange, strange
B. charm, charm, down
C. top, anti-up
D. anti-up, anti-up, anti-strange
Worked example
A K- particle has a strangeness of –1. Determine the quark structure of this particle.
Leptons
- Leptons are a group of fundamental (elementary) particles
- This means they are not made up of any other particles (no quarks)
- There are six leptons altogether:
The six leptons are all fundamental particles
- The muon and tau particle are very similar to the electron but with slightly larger mass
- Electrons, muon, and tau particles all have a charge of -1e and a mass of 0.0005u
- There are three flavours (types) of neutrinos:
- Electron-neutrinos () which are emitted with electrons
- Muon-neutrinos () which are emitted with muons
- Tau-neutrinos () which are emitted with tau particles
- Leptons interact with the weak interaction, electromagnetic and gravitational forces
- However, they do not interact with the strong force
- Although quarks are fundamental particles too, they are not classed as leptons
- Leptons do not interact with the strong force, whilst quarks do
Worked example
Circle all the anti-leptons in the following decay equation.
Lepton Number
- Similar to baryon number, the lepton number, L is the number of leptons in an interaction
- L depends on whether the particle is a lepton, anti-lepton or neither
- Leptons have a lepton number L = +1
- Anti-leptons have a lepton number L = –1
- Particles that are not leptons have a lepton number L = 0
- Lepton number is a quantum number and is conserved in all interactions
- This is helpful for knowing whether an interaction is able to happen
The lepton number depends if the particle is a lepton, anti-lepton or neither
Worked example
If the lepton number is conserved in the following decay, identify whether particle X should be a neutrino or anti-neutrino
Step 1: Determine the lepton number of all the particles on both sides of the equation
- 0 + (–1) = 0 + X
Step 2: Identify the lepton number of X
- If the lepton number must be conserved, X must also have a lepton number of –1
Step 3: State the particle X
- Particle X is an anti-neutrino